The present invention relates generally to ultrasonic measuring apparatus which emit an ultrasonic pulse towards an object having walls, receive echos reflected from said walls, and subsequently treat signals created in response to these echos, so as so determine the temporal positions of said walls.
The invention is suitable for use in medical applications, and may be used, for example, to follow the temporal movement of the position the anterior and posterior walls of a blood vessel and determine the changes in the inner diameter and in the thickness of the walls of the blood vessel with time. Whilst it will be convenient to disclose the invention in relation to that exemplary application, it is to be appreciated that the invention is not limited to that application. The invention may, for example, be used in the measurement of the thickness of the corneal lens or in the non-invasive measurement of other bodily organs.
FIG. 1 represents schematically a known manner of measuring the position of the walls of a blood vessel. This figure shows an ultrasonic transducer 1 placed above the skin 2 of a subject, which transducer 1 faces an artery 3 displayed in cross-section. The transducer 1 is controlled by an electronic circuit so as to emit an ultrasonic pulse 4, to receive ultrasonic echos resulting from the reflection of that pulse on the artery-tissue and artery-blood interfaces, and to create an echo signal in response thereto. Depending on the frequency of the ultrasonic transducer, this echo signal can represent four distinct echos 5, 6, 7 and 8, or only two echos corresponding respectively to a combination of the echos 5 and 6, and to a combination of the echos 7 and 8.
The movement of each interface is determined in the following manner. The transducer 1 emits a pulse 4 with a repetition frequency of generally between 10 Hz and 5 kHz. In order to follow the position of the echos, which are detected by the transducer after a delay which depends on the position of each interface, a temporal window of fixed size is used to define a time interval in which the echos are awaited, and which is adjusted, after each cycle, so that the echos would be detected in the center of that window if the interfaces were immobile.
Knowledge of the temporal position of each interface as well as the propogation speed of the pulse in the blood and the tissue makes it possible, by measuring the interval, to determine the change of the inner diameter and the thickness of the anterior and posterior walls of the blood vessel 3 with time.
FIG. 1 is a schematic diagram only. In practice, the echos resulting from reflection of an impulse on the anterior and posterior walls of the blood vessel 3 are not as pure, but have a much more complex form as is shown by the elemental echos E.sub.ant and E.sub.post in FIG. 2. This deformation results from the fact that the ultrasonic pulse traverses tissues having different characteristics and from the fact that the interface between the wall of a blood vessel and the surrounding tissue is not as clearly defined as, for example, the interface between a metal plate and the surrounding air.
The position of the interfaces causing these echos, notably in the medical domain, thus cannot be directly and automatically determined from the form of the echo signal.
Several techniques are known for detecting the position of the moving interfaces.
According to a first technique, the position of the interfaces is manually determined. The user displays the echo signal on an oscilloscope or another display means and choses a particular point on the echo signal onto which the echo tracker must lock. Unfortunately, this technique requires that great experience on the part of the user to determine the particular point of the elemental echo which corresponds to the position of the interface. In practice, the user choses either the peak having the greatest amplitude or the central peak of the elemental echo. There is no assurance that the point chosen effectively corresponds to the position of the interface.
A second technique consists of processing the echo signal so as to suppress the noise, only keeping the part of the signal resulting from the reflection of the ultrasonic signal on each interface. This technique, however, presents the inconvenience that the digital signal processing is not able to be realised in real time. In fact with the calculating means now available the treatment of the signals received during the opening of the temporal window takes in the order of 0.1 to 5 seconds whereas, with a pulse repetition frequency of 500 Hz, the time available for treating the echo signal, and performing associated operations such as reading and controlling peripheral devices, is in the order of 0.002 seconds.
It is necessary then to procede in two stages: in a first period, to digitize and store in real time the echo signals to be studied and, in a second period, to treat these echo signals. It can be seen that this technique presents three inconveniences, which are the necessity to have a large amount of memory, the time taken to treat the echo signals and the absence of control in real time of the data as it is acquired.
Swiss patent application no. 2871/91, which corresponds to U.S. Pat. No. 5,297,552, describes an ultrasonic measuring apparatus which addresses the problems of this second technique. That apparatus emits a first ultrasonic pulse towards a blood vessel, converts the echo signal created from the echos detected by the transducer into a series of digital values (created during the opening of the temporal window) which are then stored. In an initialisation stage, these stored digital values are treated so as to select a reference point in each elemental echo of the echo signal, determine for each elemental echo the temporal position of each interface producing that elemental echo, and calculate for each elemental echo the temporal interval between the position of the reference point of that elemental echo and the temporal position of the interface obtained by the processing.
In parallel with this treatment, an assimilation phase occurs in which the digital values resulting from the detection of echo signals from subsequent ultrasonic pulses are treated so as to track the temporal position of the reference points from each echo signal.
There follows then an acquisition phase in which the temporal position of each interface corresponding to each elemental echo of echo signals resulting from ultrasonic pulses subsequent to the assimilation phase are followed and memorized. Finally, an exploitation stage occurs during which the data memorized in the acquisition phase is used to provide information to the user, such as displaying the diameter of the blood vessel as a function of time.
It will be appreciated by those skilled in the art that various other techniques may be used in conjunction with the present invention to determine the position of the interfaces in the echo signals.
Whilst the technique described in Swiss patent application no. 2871/91 avoids the need to process each digitized echo signal so as to determine the position of each interface in each elemental echo of the echo signal in real time, some treatment of each digitized echo signal between consecutive ultrasonic pulses is nevertheless required.
In this technique each echo signal, digitized during the opening of the temporal window, needs to be treated so as to track the selected reference points on the elemental echos. In order to enable this tracking to occur, each reference point must fall within that part of the echo signal digitized during the opening of the temporal window.
Display of each echo signal received may be provided both prior to and during the intitialisation, assimilation and processing stages so that the user can verify the correct positioning of the transducer on the skin of the subject and monitor the form of the echo signals. Processing means control this display in real time. Each digital value corresponding to each echo signal needs to be read from a memory location, and adjusted for characteristics such as magnification, delay and offset. The processing means must also create a curve of "best-fit" for these digital values.
A peripheral device such as a keyboard may also be used to provide user control over the display of each echo signal and any associated graphical information which is also displayed The processing means must provide the keyboard control to enable input signals to be read from such a device. Further measuring equipment, such as a sphygmanometer, a plethysmograph or a Doppler sensor may be connected to the processing means. The signals from this equipment may be required to be sampled, read and displayed concurrently with each echo signal received from the ultrasonic transducer. In addition, the processing means must control all functions associated with the memory in which the digital values are stored.
Time is also required between consecutive pulses to perform the necessary calculations in each of the initialisation, assimilation and processing stages.
The time required to perform the above-described processing is related to the speed of the processing means and the number of values digitized from each echo signal during the opening of the temporal window. The rate at which an echo signal is sampled so as to enable its accurate reconstruction is determined by the fundamental frequency of the echo signal. In the case of echo signals resulting from the reflection of ultrasonic impulsions from blood vessels, such a sampling frequency may be 100 MHz, that is, the echo signal is sampled every 10 nanoseconds.
A humeral artery can have a diameter as small as 4 mm. The echos from the walls of a humeral artery will thus occur relatively quickly after each other. It has been found that an echo signal resulting therefrom needs only in the order of 500 digital values at a sampling frequency of 100 MHz to enable accurate reconstruction. However, a femoral or carotid artery may have a diameter in excess of 1 cm. The echos from its walls will thus be further apart, and more than 1500 digital values may be needed to accurately recover the corresponding echo signal.
The processing means used to treat these digitized values may be provided by a microprocessor, the most powerful of which available today operates at 32 MHz. The time required for an instruction cycle (often about 10 machine cycles) in such microprocessors is about 0.4 .mu.sec, and several instruction cycles are needed to treat each digital value in the manner described above. If the ultrasonic pulse produced by the transducer is repeated at a frequency of 500 Hz, less than 6000 instruction cycles may be performed between consecutive pulses.
It can be seen that for large arteries, existing microprocessors are not able to treat a sufficient number of digital values from an echo signal in the time between consecutive pulses so as to include the elemental echos produced by both walls of the artery. The tracking of the reference points on the echo signals, the display of the echo signals, the required user control and processing of the echo signal thus cannot be properly performed. Whilst a more powerful main-frame computer could be used in these applications, the much greater cost of such computers is prohibitive.